Contaminated sediments, greases, and oils, and other pollutants collect on the ground during dry periods when little or no rainfall occurs. When a storm occurs after such a dry period, the accumulated pollutants are mobilized by storm water and get flushed into surface water drainage systems. The flushing of pollutants into such drainage systems is generally undesirable, particularly if the water or other fluids flowing through such drainage systems remain untreated before being discharged into a river, lake, or ocean. The occurrence of a storm after a dry period and the corresponding flushing of pollutants into drainage systems is often referred to as a “first flush” event. First flush events are particularly troublesome in industrial areas due to the types and amounts of pollutants that accumulate.
Because the effects of first flush events are undesirable, efforts have been made to limit such effects. A common way to do so is to allow storm waters to initially flow into a detention basin and to use a flow limiting stricture to control flow out of the detention basin. Such flow limiting structures include, among others, risers, trash racks, filters, and weirs. Such structures typically try to allow sediments to settle out, prevent the outflow of surface contaminants, or prevent the outflow of larger sized pollutants.
A concern in designing such flow limiting structures is that they should not allow flooding to occur, even if preventing flooding allows pollutants to escape. As a result, flow-limiting structures are typically designed to provide for “overflow” situations during which quantities of water in excess of the design first-flush storm are allowed to flow through the structure untreated if the incoming water volume exceeds the capacity of the system. In an attempt to help prevent overflow from occurring, some structures such as perforated risers are designed to permit a higher flow rate through an outlet as water levels rise.
Unfortunately, previously known flow-limiting structures do not always provide a solution that adequately balances the design goals of preventing flooding, allowing sediments to settle, preventing flushing of surface pollutants, and limiting peak discharge flow rates. As such, there is a need for new flow limiting structures such as are disclosed herein.